The present invention relates generally to cytokine receptors, and more specifically, to Interleukin-15 receptors.
Interleukin-15 ("IL-15") is a polypeptide that was initially isolated as an epithelium-derived T-cell factor ("ETF"). Northern analysis of a variety of human tissues indicated that IL-15 mRNA is expressed by many human tissues and abundantly by placenta and skeletal muscle. Significant levels of IL-15 mRNA were also observed in other tissues including kidney, lung, liver, and heart. The best sources of IL-15 mRNA so far observed have been adherent mononuclear cells (monocyte enriched, PBM) and epithelial and fibroblast cell lines such as CV-1/EBNA mad IMTLH. Activated peripheral blood T cells (PBT), a rich source of IL-2, express no detectable IL-15 mRNA.
IL-15 shares many biological properties with Interleukin-2 ("IL-2"). These properties include proliferation and activation of human and murine T cells and the generation of lymphokine activated killer cells (LAK), natural killer cells (NK) and cytotoxic T lymphocytes (CTL). IL-15 also can co-stimulate with CD40 ligand (CD40L) proliferation and immunoglobulin secretion by B lymphocytes.
In view of the shared biological properties with IL-2, tests were conducted to determine whether IL-15 uses any of the components of the IL-2 receptor. IL-2 cell surface receptors (IL-2R) contain at least three subunits, .alpha., .beta. and .gamma. (Toshikazu et al., Science, 257:379 (1992)). Antibodies directed against the .alpha. chain of the IL-2 receptor (anti-IL-2R.alpha.) have no effect on IL-15 (Grabstein et al., Science 264:965, 1994; Giri et al., EMBO J. 13:2822, 1994). Antibodies directed against the IL-2.beta., however, are able to block the activity of IL-15, suggesting that IL-15 uses the .beta. chain of IL-2R. Similarly, some cells require the .gamma. chain of IL-2R for IL-15 signal transduction (Giri et al., EMBO J. 13:2822, 1994).
Several lines of evidence suggest that there is an IL-15 specific binding protein. For example, the IL-3 dependent murine cell line, 32D (J. S. Greenberger et al., Fed. Proc. 42: 2762 (1983)), expresses the complete IL-2R and proliferates vigorously in response to IL-2, but cannot bind or respond to IL-15 (Grabstein et al., Science in press (1994)). Similarly, early murine pre-T cells derived from day 13 fetal liver that lack CD3, CD4 and CD8 expression (TN cells) express all three IL-2R subunits, proliferate in response to IL-2, but do not bind or respond to IL-15 (Giri et al., EMBO I, in press (1994)). On the other hand, certain human cell types and cell lines (e.g., umbilical vein endothelial cells, fibroblasts and thymic and stromal cells) do not bind IL-2 but can bind IL-15 with high affinity (Giri et al., EMBO I., in press (1994)). Taken together, this information suggests the existence of an IL-15 specific binding protein.
There is approximately 96% nucleotide sequence identity and approximately 96% amino acid sequence identity between human and simian IL-15 open reading frame sequences. Similarly, there is a 81% nucleotide sequence identity and approximately 73% amino acid sequence identity between human and murine IL-15 open reading frame sequences. The diversity of biological activities ascribed to IL-15, whether differentiation, stimulation, proliferation or functional activation, is mediated by specific receptors that bind IL-15. Since the IL-15 ligand is highly conserved in a variety of species and since the human and simian IL-15 ligands bind and stimulate murine cells, it follows that IL-15 specific receptors are likely to be highly conserved in a variety of species.